Method of making electrical heating elements



Nov. 21, 1944. B. HORSFIELD METHOD OF MAKING ELECTRICAL HEATING ELEMENTS Original Filed July 24, 1940 Basil Horsfield Invnnroq vlfltiTocmLys mass... 21; 1944 ma'rnop or MAKING ELECTRICAL HEATING ELEMENTS Basil liorsfleld, Flttsburgh Pa., assignor, by mesne assignments, to Edwin. L. Wiegand Company, Pittsburgh, Pa., a corporation oi Pennsylvania Original application July 24, 1940, Serial No.

347,185. Divided and this 1941, SerialNo. 390,566

8 Claims. (CI. 18-59) My invention relates to electrical heating elements and more particularly to such elements comprising a resistor disposed in and insulated from a metallic sheath. The principal object of my invention is to provide new and improved methods of making the same. This application is a division of my application Serial No. 347,185, filed July 24, 1940.

In the drawingaccompanyin'g and forming a partof this application, I have shown, for purposes of illustration, one form of electrical heating element of the aforesaid type, and in this drawing:

Figure 1- is a plan view or an annular type of heating element, and

Figures 2 and 3 are sections taken on the lines 2-2 and 3-3 respectively of Figure 1.

The heating element illustrated. comprises. a sheet metal sheath l hereshown as of annular form. The sheath in has disposed therein a resistor ll suitably insulated from the sheath by insulating material 12, The resistor H is here shown as made of helically wound wire, but it will be apparent to those skilled in the art that it may be made of other forms of electric resistance material, wound or bent in other ways.

The sheath I0 is here shown as comprising a sheet metal channel 13 of annular form providing a plane walk, and a sheet metal cover plate It, the marginal portions it of the sides of the channel I: being bent or crimped over the cover plate I 5.

The heating element is provided with terminalsin the form of studs ll having any suitable form of heads la, the shanks ill of the studs being desirably threaded to receive nuts 20. The cover plate l5 has apertures 2| through which the shanks l9 are dispoSed, and the shanks are suitably insulated as by mica washers 22, 23.

In Figure 1 the studs 11 are shown as mounted side-by-side in suitably circumferentially spaced relation, and the cover plate l5 may have an embossed portion 24 in the region of the studs ll.

The ends 25, 26 of the resistor H are suitably connected to the studs l'l respectively, the intermediate helical portion of the resistor being disposed concentrically of the sheath in and looped back and forth a desired number of times so as to provide a desired number of strands, in this .instance four in number, it being understood that the number of strands may be any other number. One way of constructing the heating element illustrated is as follows. The resistor I I and studs II are supported on a suitable tool or form (notshown) analogous to the tool shown in the patent application April 26,

to Wiegand 1,133,347, March 30, 1935, the tool, of course, having a resistor-supporting part which is of such'shape and size that it will fit into the channel [3. The resistor ll, carried by the tool, is introducedinto an open coverless channel, such as l3 before the margins iii are crimped, the channel l3 being partly filled to a predetermined level with insulating material which is in a plastic, plastoid, or impressionable condition, and consequently the resistor l l becomes embedded in the insulating material. The tool is then withdrawn, leaving the resistor ll embedded in the insulating material. Additional insulating material is then introduced in the channel, over the embedded resistor, and the insulating material at various intermediate stages of manufacture before and after the cover I5 is applied,'and as a final treatment if desired.

The body of insulating material must of course be refractory, and it is desirable that it comprise a selected material that has suitable electrical-insulating properties, and desirably good heat-conducting properties. In instances where a bonding material has been mixed with the selected heat-conducting electrical-insulating material it has been common to use clay as the bonding material. My invention contemplates the avoidance of the use of clay.

I have found that a body of insulating material suitable for use in electrical heating elements may be made by mixing a selected electrical-insulating heat-conducting material such 1 as, for example, zircon (zirconium silicate) or other suitable material, with a bonding material made of magnesium oxide and phosphoric acid..

For example, the mixture for introduction into with the purpose of providing that'compound. For reasons hereinafterstated, the mixture of magnesium oxide and phosphoric acid shouldbe slightly acid, that is, there should be a slight excess of phosphoric acid over that necessary to react with themagnesium oxide to produce the magnesium pyrophosphate; The absolute amount of excess phosphoric acid may range from a few drops to approximately a cubic centimeter, depending somewhat upon the amount of bonding material being made up, but in any case desirably the excess is added in such amount that the acidity of the-material is such that its pH isapproximately 5 to 4 /2. The magnesium oxide and phosphoric acid are mixed to a. thin slurry which is then dried down to a sticky puttylike mass. This mass is then mixed with the zircon which desirably has been slightly dampened with water: and, preferably, talc is included in the mixture as a mechanical dilutant for the bonding material. This mixture is then used to embed the resistor as hereinbefore described.

'The amount of phosphoric acid and magv nesium oxide are adjusted, to obtain slight acidity as herelnbeforeset forth, for the purpose of securing the desired plasticity and coherence, in other words, for securing the desired workability of the mass of insulating material. It is desirable in connection with the making of electrical heating elements of the insulation-embedded resistor type that the mixture of selected heat-conducting electrical-insulating material" and bonding material have about the mechanical workability of clay in the resistor-embedding stage of manufacture, the heating element being thereafter baked or otherwise heat treated to convert the body of insulating material into a hard, firm cake.

The ingredients may be taken in the following proportions, as an example of a satisfactory batch: bonding material, comprising 260 grams of magnesium oxide mixed with 460 cubic centimetres of phosphoric acid of 85% strength, prepared as hereinbeiore described, is mixed with 4000 grams of zircon and 180 grams of talc. This mass when baked provides a cake in the finished heating element of suitable firmness and hardness. Heating elements embodying such a body of insulating material when operating at energy dissipations up to approximately 30 watts per square inch of heating surface, show a remarkably low electrical leakage when the terminals of a source of alternating current are connected to the resistor and sheath respectively, in a range'from 220 volts to a high test voltage of the order of 1200 volts. I

While excellent results are obtained by using the ingredients for the body of insulating material in the proportions hereinbefore given, that is, about l4 to 15% of the bonding material, 81 to 82% of zircon, and 3 to 4% of talc, I do not limit myself to these specific proportions; Generally speaking, the percentage of bonding material is selected to provide a cake of satisfactory mechanical characteristics for a particular use while at the same time providing a cake having the desired electrical characteristics for that particular use. I prefer to include talc because the electrical characteristics are improved. The tale is used in such sufllcient percentage as will improve the electrical characteristics of the cake but at the same time will not aaoasaa 1 insulator grade of talc orsteatite.

It is of course understood that talc when heated or burned is converted into steatite. The tale desirably should below in lime and alkali salts, and should burn satisfactorily. A pure dead white talc which is white-burning is preferred. A talc which contains a considerable percentage 0! magnetic material and iron oxide will burn a light brown and such a talc, while it may be used, is not as good as white-burning tale. The tale used in mixtures embodying my invention is desirably finely ground.

While I have illustrated one form of electrical heating element of a type of heating elements. it will be understood that my invention may be embodied in other suitableforms of that type and in other suitable types of electrical heating elements. I

From the foregoing it will be apparent to those skilled in the art that each of the disclosed embodiments of my invention provides a new and improved method 0! making electrical heating element, and accordingly, each accomplishes the principal object of my invention. On the other hand, it also will be obvious to those skilled in the art that the disclosed embodiments of my invention may be variously changed and modified, or features thereof, singly or collectively, embodied in other combinations than those disclosed, without departing from the spirit of my invention, or sacrificing all of the advantages thereof, and that accordingly, the disclosure herein is illustrative only, and my invention is not limited thereto.

I claim:

1. The method of making an electrical heating element which comprises mixing magnesium oxide and phosphoric acid inv predetermined proportions, drying the resultant mass to a sticky putty-like product of slight acidity, mixing said product in selected proportion with particles of heat-conducting electrical-insulating material to form a plastic material oi desired workability, embedding a resistor in a mass of said plastic material, and applying heat to harden said mass of plastic material.

2. The method of making an electrical heating element which comprises mixing magnesium oxide and phosphoric acid in predetermined proportions, drying the resultant mass to a sticky putty-like product of slight acidity, mixing said product in selected proportion with particles of heat-conducting electrical-insulating material and tale to form a plastic material of desired workability, embedding a resistor in a mass of said plastic material, and applying heat to harden said mass of plastic material.

3. The method of making an electrical heating element which comprises mixing magnesium oxide and phosphoric acid in predetermined proportions, drying the resultant mass to a sticky putty-like product of slight acidity, mixing said product in selected proportion with zircon to form impair the desired mechanical characteristics. I

that is, its desired hardness and firmness.

when tale is used I prefer to use an electrical a plastic material of desired workability, embedding a resistor in a mass of said plastic material, and applying heat to harden said mass of plastic material.

4. The method of making an electrical heating element which comprises mixing magnesium oxide and phosphoric acid in predetermined proportions, drying the resultant mass to a sticky putty-like product of slight acidity, mixing said product in selected proportion with zircon and tale to form a plastic material of desired workability, embedding a resistor in a mass of said plastic material, and applying heat to harden said element which comprises mixing approximately mass of plastic material.

5. The'method'of making an'electrical heating element which comprises mixing approximately 36 parts by weight of magnesium oxide with 64 parts by weight of 85% phosphoric acid, adding suflicient phosphoric acid so that the acidity of the mixture is such that its pH is approximately to 4 drying the resultant mass to a sticky putty-like product, mixing said product in selectecDproportion with particles of heat-conduct- .ing electrical-insulating material to form a plastic material of desired workability, embedding a resistor in a mass of saidplastic material, and applying heat to harden said mass of plastic material. 1

6. The method of making an electrical heating element which comprises mixing approximately 36 parts by weight of magnesium oxide with 64 parts by weight of 85% phosphoric acid, adding suflicient phosphoric acid so that the acidity of the mixture is such that its pH is approximately 5% to 4 /2, drying the resultant mass to a sticky putty-like product, mixing said product in selected proportion with zircon to form a plastic material of desired workability, embedding a resistor in a mass of said plastic materiaLand applying heat to harden said mass of plastic material.

7. The method of making an electrical heating 36 parts by weight of magnesium oxide with 64 parts by weight of 85% phosphoric acid adding 5 suflicient phosphoric acid so that the acidity of the mixture is such that its pH is approximately 5 /2'to 4 drying the resultant mass to a sticky putty-like product, mixing said product in selected proportion with zircon and talc to form a plastic material of desired workability, embedding a resistor in a mass of said plastic material, and applying heat to harden said mass of plastic material.

8.'The method of making an electrical heating element which comprises mixing approximately 36 parts by Weight of magnesium oxide with 64 parts by weight of 85% phosphoric acid, adding suflicient phosphoric acid so that the acidity of the mixture is such that its pH is approximately 5 to 4 drying the resultant mass to a sticky putty-like product, mixing said product with zircon and talc to form a plastic material of desired workability, the ingredients of said material being in the approximate proportions of 14 to 15% of said product, 81 to 82% of zircon, and 3 to 4% of talc, embedding a resistor in a mass of said plastic material, and applying heat to harden said mass of plastic material.

BASIL HORSFIELD. 

